Optical fibers are well-known devices for conducting electromagnetic radiation. A typical optical fiber will have a core in which the radiation is conducted surrounded by a cladding having a lower index of refraction. The radiation is contained within the core by total internal reflection at the interface between the core and the cladding. Some fibers have a uniform index of refraction across the cross-section. In such fibers the entire fiber functions as a core and the surrounding ambient functions as the cladding.
The most common and best known use for optical fiber is for data communications. A typical single mode fiber used for data communications has an overall diameter of 125 micrometers with a core having a diameter of 8.5 to 10 micrometers. A multimode fiber having a graded index core typically will also have an overall diameter of 125 micrometers with a core in the range of 50 to 62.5 micrometers. Other multimode fibers may have cores as large as 200 micrometers and overall diameters ranging from 230 micrometers all the way up to 1 millimeter.
Optical fiber may also be used for purposes of illumination. Although it is possible to use optical fibers having sizes similar to those used in data communications for illumination, it is preferable to use larger fibers. Larger fibers permit greater quantities of light to be conducted thereby increasing the amount of illumination possible. Optical fibers used for purposes of illumination may have overall diameters as large as a few centimeters. Furthermore, fibers used in data communications are typically either glass or glass polymer composites where the core is glass and the cladding a polymer. Fibers used for purposes of illumination are typically all polymer. This is because the polymer fibers are less expensive, especially in the large diameters used in illumination systems while the low loss characteristics provided by glass that are necessary in long distance data communications are not as important in the relatively short distances that light is conducted in a fiber used for illumination purposes.
U.S. Pat. No. 5,009,020 (Watanabe) teaches the use of optical fiber for illumination purposes in an automobile. A single light source is set in the center of a vehicle and optical fiber is used to conduct light to external lights such as headlights. The Watanabe patent does not, however, teach any system for making the light source uniform or collimating the light. Those tasks are accomplished by the light fixture into which the optical fiber inserts the light.
Numerous patents have issued to Maurice Daniel on the subject of weaving optical fibers in order to product microbends. Typical of these patents is U.S. Pat. No. 4,234,907. Some light will escape at each of these bends. A system such as this produces a very flat light source. Uniformity may be achieved by increasing the tightness of the weave as distance from the light source increases. There is no good way to increase collimation of the light, however. Furthermore, by the very nature of the weave, microbends appear on both sides of the woven layer. Therefore the light will escape from both sides of the woven mat. In most applications light is desired on only one side of the panel so this will inherently waste some of the light.
Commonly-assigned U.S. patent application Ser. No. 07/963,056 (Appeldorn et al.) U.S. Pat. No. 5,432,876, the teaching of which is incorporated herein by reference, teaches the use of notches in optical fibers to extract light. The light is extracted by total internal reflection from the notch. The level of illumination may be made uniform across the emitting area by increasing the depth of the notches as distance from the light source increases, or decreasing the spacing between notches as distance from the light source increases, or a combination of these techniques. This technique does not suffer from the defect in the Daniel woven system in that substantially all of the light is extracted on one side of the fiber. There is no teaching, however, in this patent of any way of limiting the angular range of the extracted light.